(function (doc, win) {
   var docEl = doc.documentElement,
      resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
      recalc = function () {
         var clientWidth = docEl.clientWidth;
         if (clientWidth >= 640) {
            clientWidth = 640;
         };
         if (!clientWidth) return;
         docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
      };
   if (!doc.addEventListener) return;
   win.addEventListener(resizeEvt, recalc, false);
   doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);